Bad earth
But it could also be just poor supply from the fuse board to the dash. Do they fade a lot or just a bit?
My radio front panel blinks a bit when the indicators are turned on and I don't really understand it because it get's the power supply over a 70A ignition relay supplied directly from a 6 sqmm cable to the battery! But it is probably the permanent 12V supply to the dash, which also powers the indicator relay.